This invention relates generally to the field of high speed optical sources for dense wavelength division multiplexed (DWDM) transmission systems, and specifically to a monolithically integrated light source and frequency discriminator.
The explosive growth in internet, multimedia and wireless traffic in recent years is rapidly exhausting capacity in public networks worldwide, forcing network service providers to aggressively install new lines and upgrade old ones. However, technological breakthroughs have made all-optical DWDM systems a cost-effective way to utilize the vast bandwidth already available in the embedded fiber plant.
Externally modulated lasers of either the monolithically integrated or hybrid type are high speed optical sources favored for DWDM transmission systems. DWDM based transmission systems increase the information carrying capacity of a transmission system by loading multiple channels of differing optical frequencies onto a single optical fiber. As the number of channels in a DWDM system increases in a given amplifier bandwidth, the channel spacing decreases. Over the last three years, the channel density of commercial DWDM systems has increased dramatically. As a result, the narrower frequency spacing between channels has become susceptible to long-term aging-induced frequency drifts of conventional optical sources as known in the art. Consequently to ensure that optical signals do not wander out of their allotted channel bands, and moreover, to ensure the proper performance of narrow band passive optical components which perform functions such as adding or dropping channels, optical sources must possess a high degree of frequency stability. For these reasons, frequency stabilization has become a necessary part of a DWDM transmission system.
There are several schemes for achieving the frequency stabilization of optical sources known in the art. These include the use of absolute reference cells containing vases with well defined atomic or molecular transitions, calibrated reference elements such as fiber-gratings. Fabry-Perot (FP) etalons or waveguide interferometers, and monolithically integrated reference elements such as a Distributed FeedBack (DFB) or Distributed Bragg Reflector (DBR) gratings. While these prior art schemes for the frequency stabilization of optical sources work for their intended purpose, they have significant drawback in that they are costly, not compact in size, not easy to use and do not provide the level of frequency stable, low chirp, optical signals that are required for DWDM transmission systems today and the future.
Monolithically integrated reference elements such as DUB or DBR gratings are capable of providing a single frequency optical output signal as known in the art. DFB lasers operate in a single optical mode in contrast to multi-longitudinal mode Fabry-Perot (FP) lasers. Reference is first made to FIG. 1 which depicts the optical cavity 10 of a DFB laser constructed in accordance with the prior art. DFB laser optical cavity 10 is contained within a DFB back facet 15 and a DFB front facet 16. A DFB waveguide 14 and a Bragg grating 13 extend from DFB back facet 15 to DFB front facet 16. Bragg grating 13 is a diffraction grating that provides frequency selective feedback to photons in DFB optical cavity 10. DFB optical cavity 10 is connected to a DFB signal source 11 which is generally used to pass a DC current through DFB optical cavity 10 so as to emit light in DFB waveguide 14 as known in the art. DFB optical cavity 10 is grounded by a DFB ground connection 12. A DFB output signal 18 is produced from DFB front facet 16 as known in the art. Although, a DFB laser can provide a single frequency optical signal output, it does not provide a stable frequency that is desired in a DWDM transmission system today.
An electro-absorption modulated laser (EML) is capable of providing a single frequency, high speed, low chirp optical signal output as known in the art. Reference is now made to FIG. 2 which depicts the body 20 of an EML constructed in accordance with the prior art. EMIT 20 has a modulator 19 which is connected to DFB optical cavity 10. Specifically, modulator 19 has a modulator waveguide 24 which can be constructed of substantially the same material as that used for at DFB laser waveguide as known in the art. Modulator 19 also has a modulator back side 25 and a modulator front side 26 such that modulator waveguide extends from modulator back side 25 to modulator front side 26. A modulator signal source 21 provides a signal to modulator 19. EML optical cavity 20 is grounded by an EML ground connection 22. Modulator back side 25 is connected to DFB front side 16 such that modulator waveguide 24 and DFB waveguide 14 are aligned and coextensive. An EML output signal 28 is produced from modulator front side 26 as known in the art. Although, an EML can provide a single frequency, high speed, low chirp optical signal output, it does not provide a stable frequency that is desired in a DWDM transmission system today. Thus, it is desirable to provide for a better scheme for the frequency stabilization of optical sources which is a balance of performance, reliability and cost which provides for a high speed, low chirp, single stable frequency optical source for DWDM transmission systems.
The present invention is directed at overcoming shortcomings in the prior art. Generally speaking, in accordance with the present invention, a monolithically integrated light source and frequency discriminator has a section which produces a single frequency optical signal and a section which senses the optical frequency. The light producing section comprises an active material in which an electrical signal is converted to an optical signal. A portion of the optical signal is coupled to the frequency discriminator which comprises at least two photodetectors arranged to sense the optical frequency. The outputs of the photodetectors are connected to a controller comprising a microprocessor, discrete electronics or some combination thereof, for stabilizing the optical frequency of the light source at a desired value.
In a preferred embodiment, a monolithically integrated electro-absorption modulated laser (EML) and frequency discriminator has a distributed feedback laser having a back side and a front side. A modulator is connected to the front side of the distributed feedback laser such that the modulator and the distributed feedback laser form an EML which has an output signal with an EML output frequency. Further, a two element photodetector array is connected to the back side of the distributed feedback laser such that the photodetector array can sense the EML output frequency. The photodetector is connected to controller to stabilize the EML output frequency.
Other objects and features of the present invention will become apparent from the following detailed description, considered in conjunction with the accompanying drawing figures. It is to be understood, however, that the drawings, which are not to scale, are designed solely for the purpose of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.